Alet, Pierre-Jean (2008) Hybrid thin-film solar cells based on nano-structured silicon and semiconducting polymer. PhD thesis Science des matériaux, CEA/DSM/IRAMIS/SPCSI, EP/X p.208.

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Abstract

This thesis presents an exploratory work on a new design of hybrid solar cells, which are based on a junction between an inorganic material (silicon) and a polymer (P3HT). This structure is intended to improve the efficiency of organic based solar cells while maintaining low costs. Here, we investigate its experimental feasibility, and we analyze its performance. The hetero-junction between silicon and P3HT has been studied on bilayer devices. We have shown that this junction generates electrical power under illumination, and that both silicon and P3HT can contribute to the photocurrent. Power conversion efficiencies up to 1.6 % have been obtained. A large amount of work has been done to simplify the fabrication process and to improve its reliability. Two new nano-structured silicon layers have been developed. ``Nano-sponge'' layers, where the typical dimensions of pores is 20 nm, have been obtained by metal-catalyzed plasma-enhanced CVD at 175 °C. Silicon nanowires have been grown by a completely new process: the substrates are transparent conductive oxides, the catalysts are generated in situ, and the growth temperature is below 300 °C. The würtzite (Si-IV) phase has been identified in some wires, and various growth modes are observed. Both kinds of layers may also find applications in inorganic solar cells.

Table of content

Introduction

1 Silicon/P3HT ideal hetero-junction

1.1 Introduction to the materials used in this study

1.1.1 Semiconductors for solar cells

1.1.2 Silicon

1.1.3 Organic semiconductors

1.1.4 P3HT

1.2 Potential of the silicon/P3HT heterojunction for photovoltaics

1.2.1 Junctions involving semiconductors

1.2.2 Silicon/organic heterojunction in the literature

1.2.3 Tentative band diagram

1.3 Exploration of possible multi-layer configurations

1.3.1 Design and fabrication of devices

1.3.2 Effect of the structure on photovoltaic parameters

1.4 Analysis of optimized devices

1.4.1 Diode behavior

1.4.2 Behavior under illumination

2 Real interfaces

2.1 Position of the problem

2.2 Design and fabrication process

2.2.1 Improvements on the design and the fabrication process

2.2.2 Development of a new glovebox

2.3 Interface between silicon and P3HT

2.3.1 Technical approach: prevention and characterization of the contamination

2.3.2 Analysis of the oxidation

2.3.3 Analysis of the carbon contamination

2.4 Interface between P3HT and the top electrode

2.4.1 Defective interface between P3HT and metal electrode

2.4.2 Improvement of the contact

3 TCOs as substrates for silicon nanowires

3.1 How can silicon nanowires be grown on a substrate?

3.1.1 The “top-down” approach: etching

3.1.2 The “bottom-up” approach: anisotropic growth

3.1.3 Control of the orientation and the growth direction

3.1.4 Choice and deposition of the metal catalysts

3.2 Formation of metallic aggregates on transparent conductive oxides

3.2.1 Deposition and annealing

3.2.2 Development and test of a characterization method

3.2.3 Reliability of the image analysis

3.2.4 Qualitative and quantitative evolution of the layer

3.3 CVD growth on transparent conductive oxides

3.3.1 Experimental design

3.3.2 SEM characterization of the deposited layers

3.3.3 Analysis

4 PECVD growth of silicon nanostructures

4.1 Rationale for using PECVD to grow silicon nanowires

4.1.1 Why are plasmas not widely used to grow nanowires?

4.1.2 Potential advantages of plasmas for the growth of nanowires

4.2 Nanostructured silicon on evaporated catalysts at low temperature

4.2.1 Choice of experimental conditions

4.2.2 Characterization of the catalytic effect

4.2.3 Possible growth mechanism

4.2.4 Outlook

4.3 Hydrogen plasma on evaporated catalysts

4.3.1 Hydrogen plasma treatments on copper and gold

4.3.2 Hydrogen plasma treatments on indium and aluminum

4.4 Growth of silicon nanowires with catalysts generated in-situ

4.4.1 Evidence of the growth of silicon nanowires without external catalyst

4.4.2 Crystalline structure of the wires and influence of the metals

4.4.3 Effect of the hydrogen plasma treatment on the substrate

4.4.4 Effects of the treatment time on the size and density of the wires

4.4.5 Creeping or standing nanowires?

5 Nano-structured devices

5.1 Deposition of the active layer

5.1.1 Deposition of the polymer layer

5.1.2 Deposition of the silicon layer

5.2 Performance of devices

5.2.1 Devices based on silicon nanowires

5.2.2 Devices based on silicon nano-pillars

5.3 Discussion and outlook

5.3.1 Performance and analysis of the devices

5.3.2 Interface engineering

5.3.3 Optical and electrical modeling

Conclusion

A PECVD

A.1 Chemical vapor deposition

A.2 Presentation of plasma-enhanced CVD

A.2.1 Physical characteristics of low-temperature plasmas

A.2.2 Chemistry in the plasma

A.2.3 RF-PECVD reactors

B Fabrication and characterization methods for thin-film solar

cells

B.1 Characterization methods

B.1.1 Electrical characteristics

B.1.2 Measurement methods

B.2 Fabrication techniques

B.2.1 Fabrication of multi-layer hybrid devices

B.2.2 Evaporation under vacuum

B.2.3 Operating procedure for ODILE

Notations

Bibliography

Statistiques de consultation

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